What does "use the fingerprint to validate the certificate manually" mean?

If you get a message such as the following:

Error validating server certificate for 'https://projectname.googlecode.com:443':
- The certificate is not issued by a trusted authority. Use the
fingerprint to validate the certificate manually!

...you should check that the "fingerprint" or "SHA1 fingerprint" given for the certificate is: B1:3A:D5:38:56:27:52:9F:BA:6C:70:1E:A9:AB:4A:1A:8B:DA:FF:EC. If it is not, please email us at code-hosting@google.com.